1. Field of the invention
This invention relates generally to the field of gravity measurement for the purpose of identifying subsurface anomalies that provide an indication of the presence of ore bodies, hydrocarbon bearing formations, water bearing formations, etc.
2. Background of the invention
Gravity, in the broad sense can be defined as a vector force between the earth and a mass which is attracted to the earth. My earlier U.S. Pat. No. 4,290,307, issued Sep. 22, 1981, (hereinafter my xe2x80x9c307xe2x80x9d Patent) is directed to an apparatus which measures one of the component vectors, i.e., the vertical component. Another gravity instrument developed for measurement of the horizontal vector component of gravity is disclosed in my U.S. Pat. No. 4,271,702, issued Jun. 9, 1981. Another gravity sensor is disclosed in my U.S. Pat. No. 4,756,191. This sensor of U.S. Pat. No. 4,756,191 measures the vertical component of gravity and provides a more sensitive and efficient system yielding more efficient results as compared to the inventions disclosed in my previous patents.
One phenomenon where gravity measurements of the earth are extremely helpful is in prospecting for minerals. The earth is not a homogeneous body. As a result, variations in the vertical component of gravity over a given geological region may be related to the geology of the region. As an example, large masses of iron ore create regional discontinuities in the measurements which, on proper interpretation, yield valuable information for determining the extent of the mass of iron ore in the earth.
While regional variations in gravity occur, variations also occur at a given locale over long or short periods of time as a result of a variety of reasons including, as an example, movement of extraterrestrial bodies. Accordingly, field gravity measurements need to be compared (by subtraction of time variations) to a common base station measurement (taken during the same time periods) to obtain time invariant local gravity measurements. The correlating and manipulation of the data is generally done by digital computer.
It is also important that the gravity measurement signals being transmitted to the computer take into account the temperature at which the measurement was taken. Otherwise, gravity measurements uncompensated from the standpoint of temperature fluctuations, will exhibit errors. Since my earlier patents, temperature compensation has been enhanced by conducting a series of tests under strict laboratory conditions temperatures from 20xc2x0 F. to 110xc2x0 F. to determine the effect of thermal expansion of the fluid that supports the floating unit within the meter and devising a computer program to further compensate for the expansion or contraction of the fluid supporting the floating unit.
It is an object of this invention to further reduce errors in local gravity measurements, and thereby better identify gravity anomalies.
In one embodiment of the invention, a gravity measuring instrument comprises a housing, a float, and proximity sensing means between the housing and the float. The housing defines a vertically oriented elongated chamber for containing a liquid. The chamber has an upper end and a lower end. The float is elongated and is positioned in the chamber. The float has an upper end and a lower end. A liquid partly fills the chamber and the float is positioned for free floating movement in the liquid in response to gravity variations. A proximity target is on the lower end of the elongated float and a proximity sensor is supported in immovable relation with the housing beneath the lower end of the elongated float. The sensor provides electrical signals representative of distance between the proximity sensor and proximity target.
In my U.S. Pat. No. 4,756,191 (the ""191 patent), the proximity target was attached to the top of the float device. When in use, it was discovered that the tank fluid, at times, would splash some fluid droplets atop the detection plate, thus destroying correct calibration. The instant invention overcomes this. If desired, the instant invention can also be used in combination with the groove configuration as disclosed in my ""191 patent for providing efficient temperature compensation.
My invention can be used by providing a calibrated pair of a first gravity measuring instrument and a second gravity measuring instrument, each as above described, coupled to suitable signal receiving, digitizing and processing apparatus for recording gravity readings from the electrical signals produced by such apparatus in association with time. The first gravity measuring apparatus is positioned in a first location which is fixed and a first series of gravity readings recorded against time. The second gravity measuring instrument is moved about over a plurality of second locations defining an area to be mapped for gravity anomalies and a second series of gravity readings is recorded by location against time. The first location is spaced apart from the area to be mapped but is sufficiently close to the area to be mapped that any influence exerted by celestial bodies on the first and second gravity measuring instruments at a given time is substantially the same. The existence and location of gravity anomalies in the mapped area is determined by computer processing of the first and second series of gravity readings according to a predetermined relationship between the first and second series of gravity readings.